The invention relates to how the base station in a cellular radio system allocates the frequencies, time slots and/or spreading codes available on a radio interface for the connections that need data transmission capacity between mobile stations and the base station. The invention also relates to how the allocation of the frequencies, time slots and/or spreading codes is coordinated in such base stations of the cellular radio system that are located near to each other.
A central factor in the planning of a cellular radio system is how to prevent such radio transmissions that take place in adjacent cells from interfering with each other. For instance, in a GSM (Global System for Mobile telecommunications) system, transmission and reception take place at 200 kHz wide frequency bands, the number of which is 124 in both directions. The frequency bands, or frequencies in short, are allocated for the base stations so that adjacent base stations are not allowed to use the same frequencies. The allocation of frequencies to base stations is called frequency planning, and the resulting division of the use of frequencies is called a reuse pattern. There can be defined a frequency reuse factor, which describes how far it is necessary to go from the present cell until there is found a cell at the same frequency as in the present cell. The larger the frequency reuse factor, the smaller the part of all possible frequencies that can be used in one single cell. In order to allocate frequencies in GSM and many other second-generation digital cellular radio systems, there is applied TDMA, Time Division Multiple Access, according to which a given frequency is divided into cyclically repeated time slots inside the cell. Thus one and the same frequency can accommodate several active connections inside the cell (in GSM the number is eight).
In cellular radio systems base on CDMA, Code Division Multiple Access, the transmission frequency can be the same in all cells, so that the transmissions are separated from each other by using mutually orthogonal or nearly orthogonal spreading codes. Now the above mentioned frequency planning is replaced by code planning, according to which given spreading codes are appointed for the use of the cells, so that the same codes must not be used in adjacent cells.
In third-generation digital cellular radio systems, the transmission and reception in the interface between the base station and the mobile stations is arranged in frames consisting of a variable number of slots with variable sizes. The quantity of the data transmission capacity represented by each slot is determined by the duration of the slot in question, the slot width in the frequency direction and possibly the spreading code used in the slot. From the point of view of sharing resources, the frequencies and codes in second-generation systems, as well as the slots in third-generation systems, can all be called orthogonal channels, or channels in short.
Systems based on a fixed channel reuse pattern are inflexible with respect to momentary variations in the traffic situation. If several mobile terminals are momentarily concentrated in one cell, for instance due to some mass activity, the channels allocated for said cell may run out. At the same time, there may be unused capacity in the adjacent cell, but because the coverage area of the base station of the adjacent cell does not extend as far as the heavily loaded area, the unused channels cannot be utilised.
It is possible to provide a system where adjacent cells or cells located near to each other could xe2x80x9clendxe2x80x9d unused capacity for a loaded cell. This, however, requires remarkable signalling in between the base stations, because the base stations must inform each other both of the need and availability of channels, and also be prepared to handover situations, where part of the mobile terminals of the loaded cell may move over to exactly that cell where the channels were reduced by lending them to the use of the most loaded cell.
It is an object of the present invention to provide a method and system whereby the channel allocation in the base stations of a cellular radio system can be carried out in a flexible manner. Another object of the invention is to provide a method which requires only slight signalling between the base stations.
The objects of the invention are achieved by dividing the channels allocated for each base station to priorised groups, and by taking channels or channel groups into use in the priorised order according to the demand.
The method according to the invention for controlling the use of channels in a cellular radio system base station which is in radio communication with the mobile terminals of the cellular radio system by using mutually essentially orthogonal channels is characterised in that when said channels are divided into groups with different priorities, the base station uses primarily the channel group with the highest priority.
The invention also relates to a cellular radio system characterised in that it is provided, in at least one base station, means for recording the information of the cellular radio system channels as divided into priority groups, as well as means for selectively putting to use channels from different priority groups as a response to the loading of the base station.
According to the invention, the channels allocated for each base station are divided into priorised groups. Highest in the priority order of a single base station are the channels that are as low as possible in the priority order of other base stations located in the vicinity. When the load is slight, each base station uses the channels highest in priority, so that the channel reuse factor is high and interference between cells is low. When the load increases, the base stations put to use, in addition to the channels already in use, channels that are located lower in the priority order, so that the channel reuse factor on the system level decreases. Now the possibility of interference between cells grows, but its effect can be evened out by applying frequency hopping and/or time hopping. When the load again decreases, the base stations return to use only the channels that are highest in the priority order.